The invention relates to a method for monitoring objects or an object area. An optoelectronic rangefinder has been provided which comprises a transmission device for transmitting optical signals which latter term should be understood in the broadest sense as an electromagnetic radiation either in the visible or invisible range. This rangefinder determines values of distance either from pulse timing (in a more narrow sense) or the phase relationship (Doppler system) of the transmitted optical signal, thus being a pulse timing rangefinder in the broader sense.
With such a method, it is an object of the present invention to enlarge and to improve its applicability in various practical fields. Such a method in accordance with the invetion is especially suited for fields of use, such as recording a scene of an accident or the surrounding of a criminal deed, but also for surveying objects or object spaces. A further field of use is recording cultural assets, such a buildings, or changes of an environment. The further values, which are assigned to the distance values, may be values of amplitude or passive image signals obtained from a beam splitter postponed to the scanner, or measuring values of an earlier measurement which values are combined with the actual measuring values, particularly in order to be able to discern changes of range-finder images (e.g. over time).
Just in the latter field, there will be very little change over time which is difficult to discern. The same difficulty in discerning results from movements of structures which cannot be detected visually or optically due to their contrast which is little or absent at all, as for example snow fields or scree slopes, but also in badly illuminated rooms to be surveyed. The invention with features thereof overcomes these special problems. A combination can be an additive or multiplicative one or the like, but suitably is effected by forming the difference.
The subject matter of this embodiment of the invention is to be seen in that after first scanning the object range and evaluating its data, they are stored in a memory as a reference set of data, and the sets of data of subsequent scannings are combined with the reference set of data, a differential set of data preferably being superimposed to the reference set of data or to the actual set of data, particularly in coded form, and are displayed and put out as an image on a monitor. Such a method may be used in alarm systems, on the one hand, and may serve the protection against burglary or other unauthorized intrusion or for discovering assassination attempts by deposing explosives. On the other hand, this method is able to visualize changes which develop at very slow rates, such as slides of mountain slopes or waste dumps as well as settlement phenomena in a terrain.
Applying this embodiment of the method according to the invention in alarm installations has the advantage over the well-known use of video systems that illumination of the object or object space is not necessary so that the fact of the surveillance cannot be recognized by an intruder. Moreover, the method according to the invention is insensitive to a large extent against attempts of deceiving or manipulating it, because it does not compare the structure of a surface, but rather three-dimensional images.
Certainly, known devices using a method of the above-identified type produce a so-called distance image on the screen of a monitor which indicates the respective distances from the surveying unit by mock colors. Such a distance image is a very suitable representation in many applications giving a good overview of objects distributed in depth. Of course, ouch distance image cannot or not clearly resolve structures which do not have any spatial depth at all or only a small one. For this reason, the method according to the prior art could not be applied for saving evidence after traffic accidents, because important elements of evidence, such as skid marks, splinters and other smaller parts of a car as well as marks applied by an officer to the road could not be dissolved in the distance image. Therefore, the situation after a traffic accident is usually still measured manually, e.g. by a perambulator, and is photographically recorded.
It has also been suggested to carry out measurements of an accident situation by means of a laser rangefinder first measuring points from a position the localities of which are indicated in maps or road maps, e.g. of buildings, traffic signs and so on, and then the cars involved in the accident and other objects relevant for the accident event. If with this method or with the purely manual one or other measurement has not been made, it is generally difficult, or even impossible, to determine precisely the position of certain objects afterwards.
A further disadvantage of the two methods described above is that they are extremely time consuming so that the place of an accident is blocked for a long period, thus resulting in severe traffic jams. All these special problems are solved in a simple manner by features of the invention.
Due to shading by individual objects in the space to be taken, a single recording cannot, in general, provide a complete three-dimensional information of this space. Therefore, at least two recordings are preferably made of substantially the same space from different angles of view. When subsequently evaluating the image, several identical points in the different images are marked so that an electronic calculator can generate a complete set of three-dimensional data upon the following image processing from the data of the different images. Thus, it is possible upon later evaluation to show pictures of the scene taken under any angle of view desired on a screen or print, and in particular it is possible to output a bird""s-eye view, a perspective or a parallel projection so that the evaluating officer obtains automatically a representation of the recorded space similar to an aerial photograph. Since the space coordinates to each point of the recorded space exist as a reference in the set of three-dimensional data, important image elements can be marked, e.g. by a cursor on a computer screen, in addition to a graphical display, optionally in correct scale, and the space coordinates of a point and/or the distance to any other marked point can be issued.
According to this variant of the method, generally two or three recordings are made of a place of accident. Immediately after recording and, optionally, a first control of data, the place of accident can be cleared and clearance be given for the traffic, because all sorts of evaluations and measurements can be carried from the recorded data out later.
An apparatus for carrying out the method according to the invention as provided. Using one or more high sensitive photoreceivers, it is possible to record a the scene of an accident or the like even in darkness without any expensive illumination.